The present invention relates to a lock-up control system for a torque converter of an automatic transmission for a vehicle.
Automatic transmissions including or equipped with a torque converter having a disadvantage that since there is a slip within the torque converter between the pump impeller and turbine runner even when the vehicle runs at high speeds when the torque duplication by the torque converter is not required, a loss in power transmission under this vehicle operating condition is noticeable, resulting in the poor fuel economy of the vehicle.
In order to improve the fuel economy of the vehicles having an automatic transmission of the above mentioned type, it is known to provide a lock-up mechanism, such as a direct drive clutch, within the torque converter so as to selectively engage to provide a direct drive connection between the pump impeller and turbine runner for the purpose of preventing the occurrence of the slip.
The conventional lock-up control systems are such that the lock-up mechanism is shifted into the engaged condition to provide the direct drive connection between the pump impeller and turbine runner upon a shift is made into a predetermined speed in the automatic transmission.
A known torque converter, wherein the lock-up takes place when the transmission shifts to the third speed, has defects in that the acceleration becomes poor, particularly in a vehicle of a smaller engine output, and that unpleasant vibration and noise are produced, because the lock-up takes place when the transmission may shift to the third speed even within a low load driving range and also during a low vehicle speed, thereby the acceleration from a low speed should take place with the pump impeller and turbine runner being kept in a direct connection.